GC Master Cylinder

Ever since I installed disc brakes on the rear of my Jeep Cherokee (Ford Galaxy) 5 years ago, I'd been disappointed with the braking performance. Just never seemed to be what the modification should have been. I'd already modified the proportioning valve. Well, during the Moab trip I really put the brakes to work and at times I was really concerned about my stopping power. I was determined that when I got home, I was going to resolve the issue.

In many cases the recommendation was to use a proportioning valve from a GC, but I'd never heard of anyone recommending a GC master cylinder. In my thoughts, with a master cylinder designed with 4w brakes in mind AND the proportioning valve, it should solve the problem. I knew there were some internal differences in the cylinders in the master cylinder and that the bore was 1" instead of 3/4".

So, I obtained a proportioning valve from a local JY and purchased a re-manned GC master cylinder from a local parts house ('98 GC). There were some mounting mods that was necessary to make the GC prop valve fit the XJ, but other than that, everything fit right up. I pressure bled the brake lines and started the XJ up. I had brakes, but they were right on the floor. I figured that with the larger 1" bore, I needed more travel from the vacuum diaphram, so I disassembled everything and after three seperate adjustments, I've got brakes like I've never had before.

This was one mod I waited far to long to do, but at least it's done.

What mods had you done to the stock prop. valve?

I removed the rubber cup from the piston in the proportioning valve.

When I did my rear disc swap (Crown Vic parts on my D44), I swapped in a booster, MC, and prop. valve from a Grand. Worked really well and had great balance between front and rear. I didn't try it at all with the stock MC (it was pretty much toast anyway) but I was definitely impressed with the performance from the ZJ one.

One thing that I ran into though - my XJ is a 5-speed, and after putting the ZJ booster in, it changed the pedal height enough that the pedal shaft wouldn't hit the brake light switch anymore. A small spacer cured this problem but it's just another one of those little headaches that always pops up.

The reason no one bothers with the ZJ master is that it is the same 1" bore and even carries the same part numbers! Not sure where you got the difference in bore size from??

If you have an older XJ upgrade to the dual diaphragm booster. For the master try out one for a Ford E350 van, yeah I said Ford...sorry.:doh: You'll need to adapt the fittings to work with your stock lines and slightly modify the base to mount it. The modifications for the base include opening up the mounting holes just a tidbit (real technical) and shaving a gnat hair off. Sorry for all these specs.

I've had some "rocket scientists" tell me that mathmatically this setup wouldn't give me any better bracking. I honestly don't care what anything says on paper. What I do know is that when I went with rear discs, going with the E350 MC made a huge difference. Removed the o-ring from the prop valve and braking is about spot on. Very little nose dive at all, very controlled and will lock all 4 up if I want it to.

Swapping to rear discs requires an MC with a larger bore size to accommodate the new rear calipers. The volume of an MC for disc front/drum rear is different than disc/disc.
Moving up to an MC with a bore size of 1.125 is a little large if the front brakes are still stock Cherokee, but will work OK, and be better than the stock MC. I think the best swap is the '76 Mercury Marquis with 4-wheel disc. It's almost a direct bolt in for a '94 and earlier XJ. No mods to the MC or booster, and the fittings are on the engine side of the MC. I only had to find an adapter (in my collection of spares) for the rear line, as I recall. Look here: http://napaonline.com/masterpages/N...603&Description=Brake+Master+Cylinder+-+Remfd

xjbubba said:

Swapping to rear discs requires an MC with a larger bore size to accommodate the new rear calipers. The volume of an MC for disc front/drum rear is different than disc/disc.
Moving up to an MC with a bore size of 1.125 is a little large if the front brakes are still stock Cherokee, but will work OK, and be better than the stock MC. I think the best swap is the '76 Mercury Marquis with 4-wheel disc. It's almost a direct bolt in for a '94 and earlier XJ. No mods to the MC or booster, and the fittings are on the engine side of the MC. I only had to find an adapter (in my collection of spares) for the rear line, as I recall. Look here: http://napaonline.com/masterpages/N...603&Description=Brake+Master+Cylinder+-+Remfd

Click to expand...

I disagree, after three years with rear discs I have yet to need to change master cylinders.

xjbubba said:

Swapping to rear discs requires an MC with a larger bore size to accommodate the new rear calipers. The volume of an MC for disc front/drum rear is different than disc/disc.
Moving up to an MC with a bore size of 1.125 is a little large if the front brakes are still stock Cherokee, but will work OK, and be better than the stock MC. I think the best swap is the '76 Mercury Marquis with 4-wheel disc. It's almost a direct bolt in for a '94 and earlier XJ. No mods to the MC or booster, and the fittings are on the engine side of the MC. I only had to find an adapter (in my collection of spares) for the rear line, as I recall. Look here: http://napaonline.com/masterpages/N...603&Description=Brake+Master+Cylinder+-+Remfd

Click to expand...

There is one factor you're not taking into consideration, the factory proportioning valve having the o-ring removed which allows more pressure to the rear. Fact of the matter is there are a lot of inexpensive solutions, what works for one person may not be acceptable for someone else. Trial and error, at least most of these ideas are cheap!

xjbubba said:

Swapping to rear discs requires an MC with a larger bore size to accommodate the new rear calipers. The volume of an MC for disc front/drum rear is different than disc/disc.
Moving up to an MC with a bore size of 1.125 is a little large if the front brakes are still stock Cherokee, but will work OK, and be better than the stock MC. I think the best swap is the '76 Mercury Marquis with 4-wheel disc. It's almost a direct bolt in for a '94 and earlier XJ. No mods to the MC or booster, and the fittings are on the engine side of the MC. I only had to find an adapter (in my collection of spares) for the rear line, as I recall. Look here: http://napaonline.com/masterpages/NOLMaster.aspx?PageId=470&LineCode=TS&PartNumber=101603&Description=Brake+Master+Cylinder+-+Remfd

Click to expand...

Not true with the common ZJ, Crown Vic and Explorer rear disc conversions. It is all down to the volume of the rear caliper piston compared to the volume of the wheel cylinder it replaces. Those mentioned, and the Ford Falcon ones I have, have been made to have similar volumes and in fact the Falcons had the option of drums or discs. Nothing in the whole braking system was ever changed in that case and they just bolted on one or the other to the rear axle on the assembly line.
Now it you had fitted the Chev calipers to the rear, they have a much bigger piston volume so need a bigger M/C to suit. You always run the smallest M/C while still have acceptable pedal travel. The more travel used on a smaller M/C means less effort so long as you don't end near the floor! It is just a fluid pressure verses volume equation and having worked as a brake mechanic, proved it many times over.